Fuel cells are batteries that directly convert chemical energy to electrical energy, utilizing air and oxygen as oxidants and hydrogen, methyl alcohol or ethyl alcohol as fuel. They are highly efficient, quiet, and reduce pollution. Like conventional batteries, fuel cells comprise of a separation membrane that separates the positive and negative electrodes. This separation membrane is pervious to gases but impervious to liquids and can prevent explosions that would result from the mixing of the oxidants and fuels. Catalysts on each side of the separation membrane catalyze the reaction between the positive and negative electrodes. For example, for proton exchange membrane fuel cells that use hydrogen as fuel and air as oxidants, the following reactions occur when the fuel cell operates:Negative electrode: H2→2H++2ePositive electrode: O2+4H++4e→2H2O,
During operation, the hydrogen at the negative electrode diffuses through a porous material to the catalyst layer where oxidation occurs and the hydrogen fuel loses its electrons to form protons. The electrons migrate to the positive electrode through an external circuit, while the protons migrate to the positive electrode through the separation membrane. The oxidants at the positive electrode diffuse through a porous material to the catalyst layer, accept the electrons from the external circuit, and combine with the protons that have migrated through the separation membrane to generate water.
The water content of the separation membrane in fuel cells is critical for the normal operation of the fuel cells. The protons generated at the catalyst layer of the negative electrode have to be hydrated in order to pass through the separation membrane to the catalyst layer of the positive electrode. When the proton exchange membrane has insufficient water, it is difficult for the protons to pass through the membrane. This increases the internal electrical resistance of the fuel cell and can reduce the output voltage to the extent that the fuel cell can no longer operate. Although a fuel cell produces large quantities of water during operation, the high temperature of the un-reacted gases can remove large quantities of water. If the water content in the intake gas to the fuel cell is low, the imbalance in the quantity of water in the fuel cell will result in a shortage of water in the membrane. Therefore, it is usually necessary to humidify the gas within the fuel cell to prevent dehydration of the proton exchange membrane.
One commonly used method to humidify the membrane is to directly pass the gas of the fuel cell through water at predetermined temperature such that the gas acquires humidity from its contact with water. However, this method is only suitable for small fuel cells with smaller humidifying needs as this methodology requires the use of large equipment, is inflexible, and difficult to control.
U.S. Pat. No. 5,432,020 discloses a method that externally adds the large quantity of de-ionized water that a fuel cell requires. The implementation of this method is difficult. In addition, the cost is increased for fuel cells using this method as separate replenishment of large quantities of de-ionized water is necessary.
U.S. Pat. No. 6,696,186 discloses a method to diffuse water through the membrane. This method creates a high gas resistance. In addition, it again increases the cost of the fuel cell as it requires the use of expensive ion exchange membranes.
An important application of fuel cells is as a power source for automobiles. This type of application requires that the fuel cells have an excellent dynamic response. The two previously disclosed methods for humidification have poor dynamic response properties and cannot adapt to rapid changes in power.
Due to the limitations of the prior art, it is therefore desirable to have novel methods of and devices for humidifying the proton exchange membrane of fuel cells that are inexpensive and easy to implement.